Modern integrated circuits require many layers of metallization, for example of aluminum, for both contacts to the underlying silicon active areas and for complex interconnections between the separate devices. Plasma sputtering, also called physical vapor deposition or PVD, has become the most widespread method of depositing the metallization. In metal sputtering, a target is made of the same material as will be deposited. An inactive gas such as argon is admitted into the chamber at a very low pressure and is excited into a plasma by biasing the target negatively with respect to the chamber wall or other part. An exemplary biasing voltage is -600VDC. The plasma produces positively charged argon ions which are attracted to the target at sufficient velocities to dislodge atomic sized target particles. Some of the sputter particles strike the wafer being coated and are deposited thereupon as a layer of the material of the target.
The application of sputtering to advanced integrated circuits has imposed severe requirements upon the sputtering process. One of these requirements is that the sputtered metal effectively fill narrow, deep holes. In a typical IC structure, a dielectric layer of, for example, silicon dioxide, is deposited over a previous layer, which may be the underlying silicon layer or a previous metallization layer. Holes are etched through the dielectric layer, and sputtering then deposits an aluminum layer both into the holes and over the dielectric layer. The metal in the holes selectively contact the small defined portions of the underlying layer. The metal overlayer is then photolithographically defined into horizontal interconnects. The holes are called contact holes if silicon is being contacted at the bottom or via holes if another metal layer is being contacted. For very advanced integrated circuits, contact holes and the via holes for the lower levels of metallization are very narrow compared to the dielectric thickness to provide dense circuitry. As a result, sputtering must fill holes with very high aspect ratios, defined as the depth to the width of the hole. The aspect ratio may be 5 or more for advanced integrated circuits.
Hole filling is fostered by low chamber pressure such that particles sputtered from the target do not strike any argon atoms on the direct path to the wafer. This condition is expressed as the spacing between the target and wafer being approximately equal to or less than the mean free path of sputtered atoms in the operating pressure of argon or other sputtering gas. Such a ballistic trajectory favors a sputtered particle distribution that is peaked around the normal of the wafer, that is, directed into deep hole. Furthermore, techniques are available to ionize some of the sputtered particles so as to accelerate them to the wafer in a highly aniosotropic velocity distribution to thereby effectively fill the bottom of the hole. Excessive collisions would reduce the effectiveness of the directional extraction.
Another disadvantage of a high operating pressure and frequent collisions between the energetic sputtered atoms and the argon atoms is that sufficient energy may be imparted to the argon atoms to cause some of them to be embedded in the sputtered metal film.
However, in general if the chamber pressure is reduced too much, the plasma collapses, thus stopping the sputtering process. That is, there is a minimum plasma chamber pressure. The value of the minimum pressure varies with chamber design and is increased by unnecessary loss mechanisms. In typical commercial sputtering reactors, the DC potential of the wafer is left floating although an RF bias may be applied to it for the above described directional extraction. A generally cylindrical shield placed around the sputtering volume is grounded and acts as the anode for the negatively biased target. The shield additionally acts as a coating shield to prevent the walls of the chamber from being coated with the sputtered material. The shield can be easily replaced when it becomes excessively coated. However, the grounded shield tends to extract argon ions from the plasma, that is, acts as a loss mechanism for the plasma. The loss increases the minimum chamber pressure required to support the plasma To circumvent this problem in a cost effective way, Ding et al. disclose a two-part shield, one part electrically floating, the other grounded, in U.S. patent application, Ser. No. 08/677,760, filed Jul. 10, 1996 now U.S. Pat. 5,736,021 and incorporated herein by reference in its entirety. As illustrated in the cross-sectional view of FIG. 1, the PVD reactor 10 is principally defined by a main chamber wall 12 and a target 14 of the material to be sputter deposited upon a wafer 16 supported on a heater pedestal 18 in opposition to the target 14. A vacuum pump 20 can maintain a base pressure within the reactor 12 to below 10.sup.-6 Torr, but a gas source 22 supplies an inactive working gas such as argon into the chamber at a pressure in the low milliTorr range. An electrical insulator 24 forms a vacuum seal between the main chamber wall 12 and the target so that a DC power supply 26 can bias the target sufficiently negatively with respect to the chamber wall 12 to cause the working gas to be excited into a plasma in the volume between the target 14 and the pedestal 18. An unillustrated magnetron positioned on the back of the target 14 intensifies the plasma near the target 14 so as to increase the sputtering rate. According to Ding et al., two overlapping annular shields 30, 32 are disposed between the plasma processing volume and the chamber walls 10.
One of the purposes of the shields 30, 32 is to protect the chamber walls 10 from being sputter coated with the same material coating the wafer 16. When the shields 30, 32 become excessively coated, they can be easily removed and replaced without the necessity of an in situ cleaning.
Another purpose of the shields 30, 32 is to provide electrical biasing of the sputtering process. In this configuration, the pedestal 18 is electrically floating, and the bottom shield 32 is grounded to provide a cathode to the negatively biased anode of the target 14. The negative voltage applied between the grounded shield 32 and the target 14 causes the argon working gas to discharge into a plasma, and the resultant positively charged argon ions are attracted to the negatively biased target 14 with sufficient energy to sputter atoms and other atomic-sized particles from the target 14. Some of the sputtered atoms deposit upon the wafer while others coat the shields 30, 32. The top shield 30, however, is left electrically floating, and the mobile, electrically charged plasma particles eventually charge it up to a voltage intermediate the voltages of the target 14 and the grounded shield 32. Thereby, electrical arcs between the target 14 and the shield assembly 30, 32 are significantly reduced compared to the more conventional configuration of a single grounded shield extending close to the target. The floating shield 30 serves further purposes of reducing ion loss to the walls, thereby reducing the minimum pressure required to support and plasma, and also of guiding positively charged sputtered particles toward the wafer 16.
The reactor 10 illustrated in FIG. 1 is only schematically shown, and the illustration omits the support structure for the shields 30, 32. The detailed embodiment presented by Ding et al. uses a thick floating shield 30 of stainless steel resting on a flat annular ceramic spacer or ring, which is itself supported on an inwardly extending shelf of the chamber wall 12 through a rim of the grounded shield 32. The ceramic spacer provides the required electrical isolation between the chamber wall 12 and the floating shield.
The ceramic spacer, however, presents some problems. Ceramics are brittle and prone to cracking, especially during installation. The brittleness is exacerbated for more complexly shaped bodies having sharp corners. Also, fine particles tend to flake off from ceramics. Such particles are a major problem in advanced semiconductor fabrication. Yet further, it is difficult to join ceramic and metal pieces by means of screws and the like. Metal screws promote the cracking and flaking problems described above, and they further defeat the electrical isolation provided by the ceramic. Ceramic screws are available, but they in turn are subject to cracking and flaking in the high-stress function of threaded engagement. Finally, while metal parts can be economically machined in small quantities, the fabrication of ceramic parts entails a high start up cost which becomes prohibitive for only a few specialized parts. In view of these problems, the structure of Ding et al. supporting and engaging the floating and grounded shields 30, 32 does not use screws but instead is left mechanically floating with only the weight of the somewhat massive floating shield 30 mechanically biasing the grounded shield 32 against grounded chamber wall 12. As a result, physical separations cannot be precisely controlled, and the inevitable rubbing of parts during operation is likely to produce additional particles, especially from the ceramic spacer. Also, the electrical contact between the grounded shield 32 and the chamber wall 12 is uncertain.
Polymeric insulators are available, but they are generally unable to survive in the hostile plasma environment, particularly when high temperatures are involved. Of course, metals are usually not considered to be electrical insulators.
Accordingly, it is desirable to produce a support structure for an electrically isolated part of a plasma chamber that does not require a ceramic part.
It is further desirable that such a support structure be threadably engageable with the chamber wall.